The present invention generally relates to a developing apparatus for use in image forming equipment such as an eletrophotographic copying machine, a printer, or the like, in which a developing agent is made to adhere onto an electrostatic latent image so as to visualize the image, and particularly relates to a one-component developing apparatus in which a one-component developing agent is transferred in a vibratory electric field generated between an electrostatic latent image holder and a developing agent carrier to thereby develop an electrostatic latent image.
Conventionally, a developing apparatus for visualizing an electrostatic latent image with a one-component developing agent has been realized by an apparatus in which a developing agent carrier is disposed in opposition to an electrostatic latent image holder, and a developing agent restriction member and a developing agent feed portion are provided in the surrounding of the developing agent carrier.
In such a developing apparatus, a developing agent stored in a hopper or the like is supplied from a developing agent feed portion to a developing agent carrier. The developing agent carrier is arranged so as to carry the developing agent which is made to adhere on the surface of the developing agent carrier. Further, a developing agent restriction member is provided so as to be in contact with the developing agent carrier by predetermined pressure, and the developing agent adhering on the developing agent carrier surface is restricted by the developing agent restriction member so as to be formed into a uniform thin layer, the thin layer being given desired charges. Then, the developing agent is carried to a position opposite to an electrostatic latent image holder and transferred to the electrostatic latent image holder by a vibratory electric field generated between the electrostatic latent image holder and the developing agent carrier to develop an electrostatic latent image on the electrostatic latent image holder.
In such a one-component developing apparatus, however, an image history in the thin developing agent layer formed on the surface of the developing agent carrier becomes a problem. Next, this problem will be described.
A developing agent thin layer which has been carried in the state where it adheres on the surface of the developing agent carrier contributes, in the position opposite to an electrostatic latent image holder, to development in accordance with an electrostatic latent image. At this time, however, a portion where the developing agent is consumed (a development portion) and a portion where the developing agent is not consumed are generated. In the development portion, when a developing agent carrier circulates through a developing agent feed portion, a new developing agent is fed so that a new thin developing agent layer is formed by a developing agent restriction member. In the non-development portion, on the contrary, the thin developing agent layer which has been formed passes by the developing agent restriction member again, so that the same thin developing agent layer passes by the developing agent restriction member several times if no developing agent is newly applied to a non-image portion. At this time, if the layer thickness of the developing agent on the developing agent carrier is measured, it is found that the layer thickness of the new developing agent in the development portion is thicker by several .mu.m than the layer thickness of that in the non-development portion, and the quantity of charges of the developing agent in the development portion is lower by about several C/g than that in the non-development portion. Further, adhesive force of the developing agent to the developing agent carrier in the non-development portion is larger than that in the development portion so that there occurs a difference in transfer properties of the developing agent between the non-development and development portions. These factors cause a difference in development concentration so that a fault called a ghost due to an image history appears.
For example, as shown in FIG. 7A, development is made on an image having a rectangular solid black portion 42 with long sides in the image forming direction in a range from an image forming initiation end 41a of an A4-size original 41 to a position of about 2/3 of the length of the original 41, and having a meshed portion 43 occupying approximately 50% of the residual portion of the remaining 1/3 of the foregoing area. In the thus formed image, as shown in FIG. 7B, a portion 143a of the meshed portion 143 following the rectangular solid black portion 142 in the image forming direction has higher image concentration than that of a portion 143b of the meshed portion 143 following a portion other than the solid black portion 142. This is because a developing agent on the developing agent carrier in the solid black portion, that is, in the development portion, transfers more easily than in the non-development portion, and because the quantity of charges of the developing agent in the development portion is slightly smaller than that in the non-development portion. Therefore, the quantity of developing agent for neutralizing the latent image charge of a meshed image is larger than that in the non-image portion.
A developing apparatus having means for solving the foregoing problems is disclosed, for example, in Japanese Patent Publication No. Hei-1-49945, and in Japanese Patent Unexamined Publication No. Sho-62-251771.
In the developing apparatus disclosed in Japanese Patent Publication No. Hei-1-49945, as shown in FIG. 8, a developing agent which does not contribute to development and remains on the surface of a developing agent carrier 1 is removed from the developing agent carrier 1 by a roller 107 rotating in close proximity to the developing agent carrier 1 and being supplied with a voltage for attracting the developing agent. After being transferred onto the roller 107, the removed developing agent is directly returned onto the surface of the developing agent carrier 1. In the apparatus, the image history is erased by disturbance generated when a developing agent is removed from the surface of the developing agent carrier and returned thereto again.
In the developing apparatus disclosed in Japanese patent unexamined Publication No. Sho-62-251771, there is provided a scale-off member 207 constituted by a plate-like blade contacting at its front end with the surface of a developing agent carrier 1 as shown in FIG. 9. The image history is erased by scaling off a developing agent which did not contribute to development and remains on the surface of the developing agent carrier 1.
The foregoing conventional developing apparatuses, however, have the following problems.
In the developing apparatus disclosed in Japanese Patent Publication No. Hei-1-49945, the roller 107 is provided so as to rotate in close proximity to the developing agent carrier 1, and it is therefore necessary to provide a space for disposing the roller 107.
Further, it is necessary to provide means for driving the roller 107 to rotate, means for application of a bias voltage, a blade means for returning a developing agent on the roller 107 to the developing agent carrier 1, and the like. Therefore, the developing apparatus increases in size and the mechanism is complicated, thereby increasing the cost for manufacturing.
In the developing apparatus disclosed in Japanese Patent Unexamined Publication No. Sho-62-251771, on the other hand, the scale-off means 207 is urged against the surface of the developing agent carrier 1 and a developing agent is therefore deteriorated because it is used for a long time.
Further, frictional force between the scale-off member 207 and the surface of the developing agent carrier 1 is a large load for rotational driving of the developing agent carrier 1 because the scale-off member is urged against the developing agent carrier 1, and it is therefore necessary to increase the force for rotating the developing agent carrier 1.